This application relates generally to electronic imaging, and is particularly suited for automatic focusing of electronic, medical imaging devices.
Endoscopic cameras are used, in conjunction with suitable endoscopic optics, to capture and display images of structures within the body of a subject. Such cameras commonly incorporate auto-focus mechanisms, which automatically focus the optics in order to optimize the quality of the captured image.
Various types of auto-focus mechanisms are known in the art of endoscopic imaging. For example, U.S. Pat. No. 4,389,565, whose disclosure is incorporated herein by reference, describes an automatic focusing device in which the focus of an image of the object to be observed is automatically controlled based on the distance to the object. The distance is automatically detected by varying the intensity of the light source that is used to illuminate the object and measuring the intensity of the light reflected by the object. According to the distance, a focusing lens is driven to obtain a focused image.
As another example, U.S. Pat. No. 6,749,561, whose disclosure is incorporated herein by reference, describes an auto-focusing endoscopic system, which uses a transition between an imaging area and a non-imaging area in the endoscopic image in focusing the endoscopic optics. This approach is said to allow for robust auto-focusing operation regardless of contrast characteristics of the environment.
Prior art auto-focus mechanisms systems typically set the focal distance of the imaging optics based on measurement and optimization of certain image characteristics. When the characteristics change, the auto-focus mechanism will typically seek a new focus. In some imaging environments, however, it may be difficult to find a clear optimum. As a result, the mechanism may spend a long time in seeking a focus and may then settle at an incorrect focal distance or resume seeking unnecessarily as the image changes. These sorts of problems are common, for example, in some endoscopic applications. Accordingly, there is a need for improved auto-focus methods and systems.